Fish the LA Marsh quite a bit and almost always use a yarn crab often with a rattle underneath. If the water is dirty (as it often is here) the shots are short and you need the fly to get down in a hurry.
The rattle is really effective for big black drum when they are nosing around in the mud and they can’t find anything unless they feel it or hear it.
If the water is clear then I tie on something lighter but with good profile. Usually something with alot of marabou that will kinda pulse when stripped.
I like tans, browns, rust colors but not sure that it matters a bunch.
Have had a rare day when they would not hit anything with flash on it but that is rare.
I know that the fish in the SC grass is a diff game altogether but would bet money the rattle crab would work well there.